Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

The journal's editor told the science publication Nature that the study by scientists at Britain's Newcastle University was retracted because two paragraphs in its introduction had been plagiarized.

Newcastle University blamed the plagiarism on a research associate who has left the institution, and said the science behind the research, and its conclusions, were not in question.

Experts said the plagiarism charge did not necessarily undermine the rest of the paper, though they acknowledged concerns might now be raised about the study's credibility.

The Newcastle scientists reported this month that they had produced the sperm in a laboratory and that it could one day help infertile men father children. Critics said the sperm did not have the specific shape, movement or function of real sperm.

Graham Parker, editor of Stem Cells and Development, said on the journal's Web site that the sperm study "is being retracted," without explaining why. But the scientific journal Nature quoted him as saying that half of the introduction paragraphs were plagiarized from a 2007 review in the journal Biology of Reproduction.

"This is clearly scientific misconduct," said Allan Pacey, secretary for the British Fertility Society. "I can understand why people might think, if they were sloppy here, maybe they were sloppy elsewhere."

When the initial paper was published, Pacey said he was unconvinced the cells produced could accurately be called spermatazoa.

"It was bad enough to begin with, and now we've got another scandal to deal with," he said. Pacey said he was saddened, and thought the fallout might confuse the public further and hurt scientists' credibility.

The field of stem cell research has battled controversy before, as when disgraced South Korean scientist Hwang Woo-suk claimed falsely to have created human embryos from stem cells in 2004.

Karim Nayernia, who led the sperm research at Newcastle University, was unavailable to comment.

But a statement released by the university blamed research associate Jae Ho Lee for the plagiarism, who has since left the university.

"No questions have been raised about the science conducted or the conclusions of the research," the statement said. It added that the paper will now be submitted to another academic journal and that Newcastle University will be further examining the supervision of research associates.

Elizabeth Wager, chairperson of the Committee on Publication Ethics, an international organization of publishers and editors, applauded the decision of Stem Cell and Development to retract the paper.

"This sets a line in the sand," she said. "Editors have a responsibility to correct the scientific record if misconduct has occurred."

Wager said the plagiarism charge was serious, but less worrisome than data fabrication.

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